Mobile devices that can be charged in the home, office or car may be exposed to user error, wrong supply voltages and voltage or current transients every time they are connected or disconnected. Protecting these devices from circuit damage has become a critical design issue for manufacturers of consumer electronics.
Transient Protection Design Considerations
Transient protection is critical when designing peripherals that may be powered off computer buses and automotive power buses. On computer buses, inductively-generated voltage spikes can exceed 8 V on the 5-V line and 16 V on the 12-V line, which can damage unprotected peripherals. With the advent of low-cost third-party AC-to-USB converters and car cigarette lighter-to-USB converters, the potential for transients to be seen on computer buses continues to increase.
Automotive power buses are notoriously dirty. Although they are nominally 12 V, they can range in normal operation from 8 V to 16 V. Still, battery currents can exceed 100 A and can be stopped instantly via a relay or fuse, generating large inductive spikes on the bus and increasing voltage by five times or more. With the growth of high-power electronics content in vehicles, so grows the potential for inductive spikes.
Finally, third-party power converters are increasingly common. Although these converters may filter some transients, testing by Tyco Electronics shows that their transient suppression capability varies widely. Custom-keyed power connectors are sometimes employed to prevent the use of incorrectly rated or improperly designed power supplies. This solution incurs custom tooling costs though and is not necessarily insulated from the creation of third-party adapters.
In another protection scheme, high-voltage-capable silicon solutions can be designed to provide a regulated voltage output and overvoltage lockout. The problem with the integrated silicon approach is that there is usually a significant price premium associated with this method.
In addition to the risk of connecting the wrong power supply, voltage transients can also damage a portable device. Although typical computer power supplies provide regulated lines at 5 V ± 5%, and 12 V ± 5%, under certain circumstances, the voltage at these lines may exceed 5.25 V and 12.60 V, causing potential damage to the system or unprotected peripherals. Voltage spikes can occur when there is inductance in the power bus and a rapid change in current occurs.
Polymer-enhanced Zener Diode Helps Protect Portable Electronics
The latest generation of clamping diodes, the polymer-enhanced Zener diode, offers designers the simplicity of a traditional clamping diode while obviating the need for significant heat sinking. This single-device solution helps provide protection against the use of improper power supplies, as well as transient suppression, reverse bias protection and protection from damage caused by overcurrent events.
As shown in Figure 1, polymer-enhanced Zener diode micro-assemblies incorporate a stable Zener diode for crisp voltage clamping and a resistively non-linear PPTC layer. The PPTC layer responds to either diode heating or overcurrent events by transitioning from a low- to high-resistance state. In the event of a sustained high power overvoltage condition, the tripped PPTC element limits current and generates voltage drop to help protect both the Zener and the follow on electronics effectively increasing the diode's power handling capability.
Polymer-enhanced Zener diode micro-assemblies, such as Tyco Electronics' PolyZen, are particularly effective at clamping and smoothing inductive voltage spikes. In response to an inductive spike, the Zener diode element shunts current to ground until the voltage is reduced to the normal operating range. In the case of a wrong-voltage power supply, the device clamps the voltage, shunts excess power to ground and eventually locks out the wrong supply. The relatively flat voltage versus current response of the device helps clamp the output voltage, even when input voltage and source currents vary.
The PolyZen device helps provide coordinated protection with a component that protects like a Zener diode, but is capable of withstanding very high power fault conditions without requiring any special heat sinking structures beyond normal PCB traces.
Adrian Mikolajczak, New Platform Marketing for Tyco Electronics' Raychem circuit protection products, identifies emerging requirements and circuit protection devices that simplify and better protect electronic systems. He has over 13 years experience in power component and systems design. Before joining Tyco Electronics, he led system design engineering for low power wireless devices at Texas Instruments and managed the high frequency microwave amplifier business at Watkins Johnson. He has a Bachelors of Engineering in mechanical engineering and took part in the EE masters program at Stanford University. He can be reached at 650-361-5114 or firstname.lastname@example.org.